Arvind Narayanan's journal

Due to popular demand one comment, I'm updating again. I don't have anything very interesting to say right now, so I'll throw out a cute puzzle I came across recently.

There are four people trying to cross a bridge. The bridge can only carry two at a time. The four have one flashlight between them. No one can be on the bridge without a flashlight. The four take 1, 2, 5 and 10 minutes respectively to cross (if more than one cross at the same time, they cross at the speed of the slowest). How can they all cross in no more than 17 minutes total?

If you think it's impossible, keep thinking. (There is no catch :-)

I'm guessing the straightforward generalization of the problem is NP-complete, but I haven't thought about it.

I am guessing that perhaps it can be solved by greedy appproaches. If not, then its probably an interesting problem for a STOC/FOCS paper (prove NP-complete, prove hard to approximate using PCP etc) :)